Petroleum is currently estimated to account for over 35% of the world's total commercial primary energy consumption. Coal ranks second with 23% and natural gas third with 21%. The use of liquid hydrocarbon fuels on an enormous scale for transportation has led to the depletion of readily accessible petroleum reserves in politically stable regions and this, in turn, has focused attention, economically, technically and politically on the development of alternative sources of liquid transportation fuels. Liquid hydrocarbons are far and away the most convenient energy sources for transportation in view of their high volumetric energy. The energy density of gasoline, for example at about 9 kWh/litre and of road diesel at about 11 kWh/litre, far exceeds that of hydrogen (1.32 kWh/litre at 680 atm, or batteries, 175 Wh/kg. Furthermore, the liquid hydrocarbon fuel distribution infrastructure is efficient and already in place.
Production of liquid fuels from biomass can help solve the problem of CO2 emission from the transportation sector because CO2 released from vehicle exhaust is captured during biomass growth. While direct, carbon-neutral use of biomass as fuel is established, for example, biodiesel, this route is limited because the limited choice of source materials, e.g. vegetable oils. Conversion of a wider variety of biomass sources into more traditional types of fuel, principally hydrocarbons, is the more attractive option.
Currently, there are two major routes for conversion of biomass to liquid fuels: biological and thermo-chemical. In the biological process, fermentation of easily fermentable plant products, such as, for example, sugars to alcohols is achieved. These easily fermentable plant products can be extracted from corn kernels, sugar cane and etc. The major disadvantage of this pathway is that only a fraction of the total carbon in biomass is converted to the final desired liquid fuel. It has been calculated that conversion of all corn produced in USA to ethanol can meet 12% of entire US demand for gasoline which reduces to 2.4% after accounting for fossil fuel input required to produce the ethanol.
One well-established route to the production of hydrocarbon liquids is the gasification of carbonaceous materials followed by the conversion of the produced synthesis gas to form liquids by processes such as Fischer-Tropsch and its variants. In this way, solid fuels such as coal and coke may be converted to liquids. Coal gasification is well-established, being used in many electric power plants and the basic process can proceed from just about any organic material, including biomass as well as waste materials such as paper, plastic and used rubber tires. Most importantly, in a time of unpredictable variations in the prices of electricity and fuels, gasification systems can provide a capability to operate on low-cost, widely-available coal reserves. Gasification may be one of the best ways to produce clean liquid fuels and chemical intermediates from coal as well as clean-burning hydrogen which also can be used to fuel power-generating turbines or used in the manufacture of a wide range of commercial products. Gasification is capable of operating on a wide variety of input materials, can be used to produce a wider variety of output fuels, and is an extremely efficient method of extracting energy from biomass. Biomass gasification is therefore technically and economically attractive as an energy source for a carbon constrained economy.
The conversion of biomass to hydrocarbon transportation fuels by the gasification-liquefaction sequence has, however, certain limitations both technically and economically. First, the conversion of the biomass to synthesis gas requires large process units, high in capital cost to deal with the enormous volumes of gas generated in the process. Second, the gas-to-liquid conversion uses catalysts which may, for optimum results, use noble metal components and accordingly be very expensive. Third, and by no means least is the fact that enormous biological resources are needed to supply current consumption levels. An approximate estimate for the land area required to support the current oil consumption of about 2 million cubic metres per day by the US transportation sector is of the order of 2.67 million square km which represents 29% of the total US land area, using reasonable assumptions for the efficiency of the conversion process, thus suggesting that large scale production of liquid fuels from such a biomass conversion process is impractical. Substitution of a part of the transportation fuel demand by biological materials would, however, constitute a worthwhile economic, political and environmental advance.
Biomass oil provides one of the options which are being considered as a source of synthetic petroleum substitutes for fuel uses. It may be extracted by biomass-to-liquid technology involving destructive distillation of dried biomass in a reactor at temperature of about 500° C. with subsequent cooling. Biomass oil produced by rapid pyrolysis has been produced commercially on a small scale. Pyrolysis oil is a kind of tar and normally contains high levels of oxygen which preclude it from being considered as a direct hydrocarbon substitute. It is hydrocarbon insoluble, viscous, contains upwards of 20 wt % water along with 40-50 wt % organic oxygen compounds that decrease the heating value, and is unstable because sediment is formed via e.g., phenol-formaldehyde resin forming reactions that lead to coke formation on heating. Biomass oil produced by hydrothermal liquefaction is a higher grade hydrocarbon soluble oil with only about 15 wt % oxygen-containing organic compounds. Previous attempts to commercialize this approach have failed due to the high water usage and inability to feed the biomass effectively into the processing unit.